There has been known an electrophotographic image forming device that includes a detachable cartridge having an agitator for agitating toner and that forms images by supplying toner accommodated in the cartridge to a photosensitive drum. In this type of image forming device, a remaining toner amount in the cartridge may be estimated based on a light-receiving signal obtained by detecting a light that has passed through windows on both side walls of the cartridge, and replacement time of the cartridge may be determined based on the estimated toner amount.
Some type of image forming device has a normal mode in which images are formed on such normal recording sheets as plain paper sheets and a low-speed mode in which images are formed on envelops, thick sheets, or the like, with the photosensitive drum and the like rotating slower than in the normal mode. The photosensitive drum, the agitator, and the like of the image forming device are configured to drive in conjunction with one another. Thus, when the rotation speed of the photosensitive drum is increased, then the rotation speed of the agitator also increases. When the rotation speed of the photosensitive drum is lowered, on the other hand, then the rotation speed of the agitator also decreases.